1. Field of the Invention
The present invention relates to a combustor for a gas turbine engine, and, more particularly, to a triple annular combustor for a gas turbine engine having extremely low emission levels without the aid of water or steam injection.
2. Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions, especially for industrial and power generation applications. Nitrous Oxide (NOx), which is a precursor to atmospheric pollution, is generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. Reductions in gas turbine emissions of NOx have been obtained by the reduction of flame temperatures in the combustor, such as through the injection of high purity water or steam in the combustor. Additionally, exhaust gas emissions have been reduced through measures such as selective catalytic reduction. While both the wet techniques (water/steam injection) and selective catalytic reduction have proven themselves in the field, both of these techniques require extensive use of ancillary equipment. Obviously, this drives the cost of energy production higher. Other techniques for the reduction of gas turbine emissions include "rich burn, quick quench, lean burn" and "lean premix" combustion, where the fuel is burned at a lower temperature.
In the typical aircraft gas turbine engine, flame stability and variable cycle operation of the engine dominate combustor design requirements. This has in general resulted in combustor designs with the combustion at the dome end of the combustor proceeding at the highest possible temperatures during stoichiometeric conditions. This, in turn, leads to large quantities of NOx being formed in such gas turbine combustors since it has been of secondary importance.
In a typical industrial gas turbine engine, fuel is burned in a single annular combustor. At maximum power, the combustor dome region is very rich with fuel, and the local fuel-air ratios can be as high as one and one-half times the stoichiometric value. These high fuel-air ratios result in extremely high flame temperatures which provide rapid combustion and excellent flame stability. However, the high flame temperature also causes the formation of large quantities of NOx resulting from oxidation of the atmospheric nitrogen since the rate of production of NOx is an exponential function of the flame temperature. Because the dome fuel-air ratio of a conventional combustor is so high at full power, the combustor is still capable of good performance at low power where the fuel-air ratios are much lower.
Further, approximately one-third of the total combustor air available is typically used in the dome region for combustion. A large portion of this total combustor air (approximately one-fifth) is used to film cool the dome region. This large quantity of cooling air is acceptable for good performance with the rich primary zones of conventional combustors, but results in increased low power emissions such as carbon monoxide (CO) and unburned hydrocarbons (UHC) as well as potential early flame blowout. It should be noted that by "total combustion air" it is meant all of the compressed airflow supplied to the combustor from the diffuser except that allocated for cooling the turbine.
The combustor of the present invention pertains to the design of a dome and fuel technique utilizing the lean premixed concept of fuel and air during all stages of operation. In order to provide low fuel-air ratios at maximum power (where the fuel flow rate is high), the combustor of the present invention utilizes approximately 80-90% of the total combustion air in the dome. Further, the volume of the present combustor was increased over traditionally compact aeroderivative combustors in order to increase residence times to achieve low CO and UHC. Accordingly, a large number of fuel/air mixers is required to accommodate this high dome flow. This is accomplished by providing three domes or banks having a number of fuel/air mixers positioned therein. Consequently, the combustor is able to operate in a temperature range which minimizes NOx, CO, and UHC.
Accordingly, a primary objective of the present invention is to provide a combustor for a gas turbine engine which produces reduced levels of emissions without the aid of water or steam injection.
Another objective of the present invention is to provide a combustor which can accommodate an extremely high percentage of the total combustion air to provide low fuel-air ratios at maximum power.
Yet another objective of the present invention is to provide a combustor which has the flexibility to accommodate various schemes for staging fuel therein.
Another objective of the present invention is to provide a combustor which minimizes the nonuniformity of combustor air supplied to the fuel/air mixers.
A further objective of the present invention is to provide a combustor which can be easily assembled and disassembled for maintenance purposes.
Still another objective of the present invention is to provide a combustor which utilizes mixers that maximize mixing of fuel and air.
Another objective of the present invention is to provide a combustor which minimizes the amount of cooling air that enters the combustor primary zone to prevent quenching the lean combustion gases.
These objectives and other features of the present invention will become more readily apparent upon reference to the following description when taken in conjunction with the following drawing.